WASHINGTON -- U.S. President Barack Obama and British Prime Minister David Cameron have reaffirmed Afghan troop-withdrawal plans and sent a stern message to Iran over its nuclear activities in a meeting at the White House.

At a joint press conference after their meeting, the leaders said significant progress had been made in stabilizing Afghanistan. Obama said plans were still on course for withdrawing all troops from the country by the end of 2014.

"[The] transition is already under way, and about half of all Afghans currently live in areas where Afghan security forces are taking responsibility," Obama said. "Today the prime minister and I reaffirmed the transition plan that we agreed to with our coalition partners in Lisbon."

The burning of Korans at a U.S. base in Afghanistan last month and the killing of 16 Afghan civilians allegedly by a U.S. soldier this week have raised questions about the pace of withdrawal.

Obama said he also expected to have a U.S.-Afghan strategic partnership agreement formalized by May's NATO summit, where the withdrawal timetable will be finalized.

The United States and Britain are the two largest contributors to the NATO-led International Security Assistance Force in Afghanistan.

Iran's Diplomatic Window 'Shrinking'

The two leaders' talks also focused on Iran, which the West has hit with stiff economic sanctions over its suspect nuclear program, which Tehran insists is peaceful.

Obama said that while every effort would be made, Iran "should understand that because the international community has applied so many sanctions [and] because we have employed so many of the options that are available to us to persuade Iran to take a different course, that the window for solving [the nuclear] issue diplomatically is shrinking."

Cameron urged Tehran to take seriously upcoming talks with world powers over the country's nuclear program, while insisting that "nothing is off the table" to resolve the crisis.

Russia On Syria

The two leaders also discussed the crisis in Syria, where the United Nations estimates that more than 8,000 people have been killed in President Bashar al-Assad's crackdown on the population.

Cameron said Russia, which has opposed UN action against the regime, would be well-advised to change course.

"In the case of Russia, I think we should also appeal to their own interests. It's not in their interest to have this bloodied, broken, brutal regime butchering people nightly on the television screens," Cameron said.

"The irony is that people in Syria often felt that the Russians were their friends and many in the West they were more suspicious of," he added. "Now they can see people in the West wanting to help them, raising their issues, calling for the world to act on their problems -- and we need to make sure that Russia joins with that."

When asked by a reporter if the leaders had discussed the possibility of enforcing a no-fly zone over Syria, as Western powers did during last year's uprising in Libya, Obama said he did not favor military intervention. He added, "Our military plans for everything."